The present invention relates to the design of a MEMS based micromixer and more specifically with the use of acoustic energy to mix very small quantities of fluid.
Microfluidic processing systems need to transport and/or mix two or more kinds of fluids of accurately controlled amount in reasonable period of time. Since many microfluidic devices are fabricated in planar lithographic environment, most of the macroscopic approaches for fluid mixing like turbulence and mechanical actuation are inapplicable at microscopic levels. and using heat for mixing is not desirable for mixing temperature sensitive fluids (such as a DNA sample). A mechanical plunger with a push-pull operational mode is effective for mixing fluids, but only as long as the fluid height is greater than 500 xcexcm while the fluid-surface area is around mm2-cm2 range.
It has been reported that focused acoustic waves (generated by annular rings of half-wave-band sources made of piezoelectric thin film and electrodes sitting on a diaphragm) are effective in generating fluidic motion. But when the fluid height is reduced to 100 xcexcm range while the fluid-surface area remains in mm2-cm2 range, there must be much stronger lateral acoustic pressure to push and pull the fluid for mixing. Thus, the problem of successfully, selectively mixing fluid in very small amounts remains.
It is an object of the invention to provide method and apparatus for micromixing of very small fluid amounts.
It is a related object to the invention to provide micromixing by utilizing acoustic wave generation and an acoustic transducer.
In summary, the present invention utilizes a Fresnel Annular Sector Actuator (FASA) for micromixing of fluids. The FASA is based on a self-focusing acoustic wave transducer which focuses acoustic waves through constructive wave interference. In the transducer, RF power is applied between the electrodes (sandwiching a piezoelectric film) with its frequency preferably corresponding to the thickness mode resonance of the piezoelectric film. Strong acoustic waves are generated over the electrode area, and interfere with each other as they propagate in the fluid. By proper design of the electrodes, and forming various combinations of the electrodes, wave. focusing can-be achieved. The mixing can be further enhanced by providing selective actuation of the different segments.
More specifically, it has been observed that when a complete annular ring is broken into segments at different angles, there are proportionate changes in the vertical and lateral acoustic potential profile. With the angle of the sector profile getting smaller, the gradient of the lateral acoustic potential becomes greater. At the same time, the vertical potential profile becomes more distributed. The present invention, while useful with sector profiles of various angles less than a complete ring, is optimized in the region of around a 90xc2x0 segment. The electrode pattern of this transducer has a high lateral acoustic potential across the focal plane of the device. The pattern is preferably realized by patterning aluminum on both sides of a piezoelectric substrate. When RF power is applied between the electrodes which frequencies correspond to the thickness mode resonance piezoelectric substrate, acoustic waves are generated.
More specifically, the electrode patterns are designed to produce constructive wave interference by utilizing a RF signal source. The RF can also be modulated using a high speed switch, or by a pulse generator. The modulated RF signal is then amplified and fed in the sector device. This causes a strong lateral force in the liquid at the focal plane. By energizing different designs of using a plurality of FASA elements, including a single overlap design which has segmented top and bottom electrodes such that the overlap area under actuation at any given time is 90xc2x0; or a four sector corner design providing for isolated sectors placed away from the center and four cornered sectors to eliminate dead zone at the comers, and a six sector corner design where additional segments are added in the middle for more area coverage. By periodic actuation using appropriate electronic controls of these sectors strong fluid flow in different direction is generated. By proper control in the actuation, a random or controlled mixing is achieved. This is realized by time phasing the sectors. Finally, by varying the time frequency of the sectors in accordance with different fluid characteristics, a wide range of fluid mixing can be accomplished.